The present invention relates to novel spray-dried catalyst compositions for use as heterogeneous olefin polymerization catalysts. In particular, the invention provides spray-dried catalyst compositions that are capable of producing olefin polymers, especially homopolymers of ethylene or copolymers of ethylene and one or more C3-10 α-olefins, having a desirable high molecular weight fraction.
Spray-drying techniques have been applied to catalyst compositions, for example, as an alternative to impregnating the catalyst on a support. For example, U.S. Pat. No. 5,290,745 disclosed preparing a solution of titanium trichloride and magnesium dichloride in an electron donor compound (for example, tetrahydrofuran), admixing the solution with a filler, heating the resulting slurry to a temperature as high as the boiling point of the electron donor compound; atomizing the slurry by means of a suitable atomizing device to form droplets, and drying the droplets to form discrete solid, catalyst particles.
It is also known from the teachings of EP-A-449,355, WO93/19100, Research Disclosure 218028-A, and WO93/11166 to prepare particles of MgCl2, optionally containing a controlled quantity of residual alcohol by spray drying alcoholic solutions of magnesium dichloride. The resulting product is used to prepare supported catalysts by contacting with TiCl4 or other titanium containing complex forming compounds.
Despite the advances in the art occasioned by the foregoing procedures, the polymer products resulting from the use of the foregoing spray dried catalyst compositions are often of rather narrow molecular weight distribution, and/or lacking in a desirable high molecular weight component. In addition, the polymers resulting from use of the foregoing catalyst lack a highly desirable product uniformity, are often deficient in molecular weight, and generally are formed in limited productivity.
Accordingly, there is an ongoing need for providing spray-dried catalyst compositions that are capable of producing olefin polymers having a desirable portion of high molecular weight component and/or a broad molecular weight distribution. In particular, there is a continuing need to provide spray-dried catalyst compositions comprising a magnesium dichloride support and a homogeneous mixture of more than one transition metal compound, especially a mixture of titanium- and hafnium chloride compounds. The compositions and spray-drying methods of the present invention satisfy these needs.